1. Field of the Invention
The present invention relates to a one-way rotational transfer mechanism having a rotary input shaft and a rotary output shaft which are concentrically arranged, wherein rotation of the rotary input shaft is transferred to the rotary output shaft when the rotary input shaft is rotated, but rotation of the rotary output shaft is not transferred to the rotary input shaft when the rotary output shaft is rotated. The present invention further relates to an ultrasonic rotary drive device incorporating this one-way rotational transfer mechanism.
2. Description of the Related Art
An example of a one-way rotational transfer mechanism having a rotary input shaft and a rotary output shaft which are concentrically arranged, wherein rotation of the rotary input shaft is transferred to the rotary output shaft when the rotary input shaft is rotated but rotation of the rotary output shaft is not transferred to the rotary input shaft when the rotary output shaft is rotated, is disclosed in, e.g., Japanese unexamined patent publication H08-177878 (hereinafter referred to as Patent Document 1).
The one-way rotational transfer mechanism disclosed in Patent Document 1 is provided with: an outer ring (rotary input member) and an inner ring (rotary output member) which are provided as concentrically-arranged ring members; a retainer positioned between the outer ring and the inner ring to be rotatable relative to both the outer ring and the inner ring (the retainer rotates in the same rotational direction as the outer ring while trailing behind the rotation of the outer ring when the outer ring rotates); and a plurality of rollers (torque transfer members) held in a plurality of pockets (through holes) to be rotatable therein which are formed in the retainer along a circumferential direction. The outer peripheral surface of the inner ring is formed in a cylindrical shape, and the outer ring is provided on the inner peripheral surface thereof with a plurality of cam surfaces (circumferentially-uneven-width grooves) having different radial depths at different circumferential positions which are arranged in a circumferential direction.
In this one-way rotational transfer mechanism, since a rotation of the outer ring in one rotational direction causes the retainer to rotate in the same rotational direction while trailing behind the rotation of the outer ring, each roller firmly wedges between one of the circumferentially opposite ends of the associated cam surface and the outer peripheral surface (cylindrical surface) of the inner ring. Thereupon, the rotation of the outer ring is transferred to the inner ring via the plurality of rollers, so that the inner ring rotates in the same rotational direction as the outer ring while trailing behind the rotation of the outer ring. On the other hand, even if the inner ring is rotated, the rotation of the inner ring is not transferred to the outer ring because this rotation of the inner ring is not transferred to the outer ring via the outer peripheral surface (cylindrical surface) of the inner ring (each roller is not moved to either of the circumferentially opposite ends of the associated cam surface).
The one-way rotational transfer mechanism disclosed in Patent document 1 requires the aforementioned retainer that rotates relative to the outer ring (rotary input member), which increases the number of elements of the one-way rotational transfer mechanism and complicates the structure thereof.
However, if the retainer is omitted from the one-way rotational transfer mechanism disclosed in Patent document 1 (or if the cylindrical outer peripheral portion of the retainer (in which the aforementioned plurality of pockets are formed) is omitted from the retainer), the plurality of rollers would be freely movable between the inner ring and the outer ring, and consequently, the plurality of rollers would move asynchronously with one another. In the case where the plurality of rollers move asynchronously with one another in this manner, in a state where each roller firmly edges between one of the circumferentially opposite ends of the associated cam surface and the outer peripheral surface (cylindrical surface) of the inner ring, rotating the outer ring in the rotational direction opposite to the previous rotational direction of the outer ring (rotational direction of the outer ring immediately before each roller firmly edges between the one circumferential end of the associated cam surface and the outer peripheral surface of the inner ring) causes each roller to be disengaged from therebetween asynchronously with other rollers, i.e., to be disengaged from therebetween at different times. If the plurality of rollers are disengaged at different times in such a manner, it takes time until the wedging force (contacting force) completely disappears, which makes it difficult to rotate the outer ring smoothly in the reverse rotational direction.